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Foreign Service

This is the fourth part of a series on troubleshooting Toyota’s most common vacuum-operated EGR systems. Before diving into this material, be sure you’ve read last month’s Foreign Service.

This is the fourth part of a series on troubleshooting Toyota’s most common vacuum-operated EGR systems. Last month, I discussed the operation of Toyota’s “constant-vacuum” EGR system, which appears on its popular 3.0L V6 (1MZ-FE) engines. This time I’ll cover troubleshooting tips for the familiar DTC P0401 (insufficient EGR flow) on this system. Be sure you’ve read last month’s Foreign Service column before diving into this material. It can be found at www.motormagazine.com.

I have emphasized using a simple, sensible process of elimination in previous columns; I’ll do the same this time. Yeah, it’s neat when you have a scan tool that performs “active” or actuator tests on Toyota systems, but it’s not absolutely necessary here. Anyway, begin by getting an accurate vehicle history and then performing a road test. Next, carefully inspect the EGR system visually.

If these steps don’t uncover any trouble, isolate the EGR valve and passages from the rest of the EGR system. To do this, disconnect the vacuum hose from the EGR valve while the engine is idling at operating temperature. Then slowly apply vacuum directly to the valve with a hand-held vacuum pump. Typically, this EGR valve begins opening by about 2.0 to 3.0 in./Hg. You usually can hear and feel the engine misfiring by the time you’ve applied 3.0 in./Hg to the EGR valve on these 3.0L V6 powerplants.

Apply still more vacuum and the engine should either stall or idle very rough. If it does either, this verifies that the valve works and the EGR passages aren’t blocked. Plus, the EGR valve diaphragm should hold vacuum.

I prefer the tried-and-true technique of manually opening the EGR valve and checking the engine’s response to it. But monitoring the EGR temperature sensor is another way to tell if exhaust is recirculating—and if the temp sensor itself is working. At the very least, it should react correctly to our manual EGR test. This sensor threads into the lower leg of the EGR assembly and its electrical connector is near the throttle lever. You can backprobe its connector and read its reaction on a voltmeter or monitor it with a scan tool. Usually, the EGR temp signal is approximately 4.00 volts when cold and may go below 1.00 volt when hot.

For example, I recently tested a healthy 3.0L V6 both before and after the EGR system really warmed up. I used a throttle tool to maintain a steady 2000 rpm so the engine wouldn’t stall when I opened the EGR valve. Then I connected a voltmeter to the EGR temperature sensor and opened the EGR valve with a vacuum pump. Within 30 seconds, the sensor signal dropped from 4.10 volts to 2.78 volts. I repeated the test after the engine had been thoroughly road-tested. Opening the EGR valve dropped the temp sensor signal from 1.44 volts to .90 volt.

If everything has checked okay so far, you’ve successfully separated the “delivery” side of the EGR system from the “control” side. Therefore, the root cause of the DTC P0401 is somewhere in the control side of the EGR system. In previous columns on Toyota EGR, I said that the control side of the system routes control vacuum to the EGR valve. The maximum available control vacuum on Toyota’s constant-vacuum EGR system is 5.0 in./Hg.

Furthermore, our test results at this point suggest that the EGR valve isn’t receiving any control vacuum. You can test this the same way we did on the vacuum-modulator EGR system (September 2009). That is, tee a vacuum gauge into the hose right at the EGR valve, then do a brief road test. Typically, control vacuum ranges from 2.0 to 4.0 in./Hg during light-throttle driv-ing. Understandably, it varies according to engine load, rpm and feedback from the EGR position sensor.

Last month, I pointed out that the entire EGR system is right on top of this 3.0L V6 engine. So if you verify that there’s no control vacuum at the EGR valve, it’s easy to work your way back to the manifold vacuum source on the engine. There are three items in series here: the little round vacuum reservoir, the EGR Vacuum Switching Valve (VSV) and then the Vacuum Control Valve (VCV). I’ll talk about that EGR position sensor on top of the EGR valve in a moment.

The VCV is the round, purple component with two hoses connected to it. When the engine is running, there should be manifold vacuum at the hose going into the center of the VCV. Meanwhile, there should be a constant 5.0 in./Hg coming out of the VCV through the hose that’s offset to one side. Yes, I mean a constant value, regardless of rpm. Replace the VCV if it doesn’t maintain the correct vacuum level at all times.

Let’s get back to that EGR VSV. Battery voltage goes to one of its two terminals. The ECM turns it on by switching the other terminal to ground. Remember three VSV details here: First, the VSV vents off control vacuum until the ECM begins turning it on. Second, the ECM turns on the VSV by switching or toggling it to ground. Third, when the ECM pulses the VSV on and off during EGR operation, it’s pulsing the VSV vent open and closed. Closing the vent, in turn, routes control vacuum (maximum of 5.0 in./Hg) over to the EGR valve.

If your scan tool performs actuator (“active”) tests on Toyota vehicles, it saves time by allowing you to momentarily bypass EGR-related ECM inputs. These inputs include coolant temperature, rpm, airflow, EGR temperature and EGR valve position. In this case, use the scan tool to turn on the EGR VSV and then observe the engine’s reaction as well as the response of the EGR temperature and position sensors.

But that said, you also can do practical checks with basic, generic tools. For instance, start the engine and check voltage at both EGR VSV terminals at idle. If you have battery voltage at both terminals, at least you know supply voltage has reached the VSV and its windings have not open-circuited.

Next, grab some long test leads. Connect a duty cycle meter or oscilloscope to the ground terminal of the EGR VSV and road-test the vehicle. During normal EGR operation, you should see a duty cycle reading or a digital scope pattern here. If you do, then you know the ECM is doing its job—toggling the VSV on and off in order to operate the EGR system. Now add up the results of the quick checks I’ve covered so far. If the ECM is toggling the EGR VSV but no control vacuum reaches the EGR valve, then it’s time to replace that VSV. But if the computer isn’t pulsing the VSV, then you have to check the VSV-to-ECM circuit and then the relevant inputs to the ECM.

You can simplify the entire troubleshooting process by combining two tests on the same road test. That is, check for control vacuum at the EGR valve and for electrical activity at the ground side of the EGR VSV. If the ECM commands the EGR VSV to act, then you’d better see control vacuum at the EGR valve.

EGR Position Sensor

I described the EGR position sensor in my last column. Typically, this sensor’s signal is approximately 1.00 volt on a closed EGR valve and around 4.00 volts on a wide-open one. These sensors have been known to stick. Tom Schilder at Motorcar Makeovers in Blue Bell, PA, cautioned me that he’s traced the P0401 code on these V6 engines to a sticking EGR position sensor. For instance, Schilder said he has verified that the EGR valve actually was closed at idle. But in the closed-valve position, the sensor reported about 2.50 volts instead of 1.00 volt. Replacing the sensor fixed the car, he said.

Finally, Schilder explained that the simplest way to locate a replacement position sensor is to give your favorite Toyota parts person the part number on the sensor itself. Otherwise, he or she may not find a listing for an EGR position sensor!